Field oriented control (FOC) of an electric motor utilizing a current regulator is widely employed in industrial electric drive systems. Electric motors utilized in electric power steering (EPS) applications are generally required to produce very low torque ripple. Thus, the torque ripple produced by an interior permanent magnet synchronous motor (IPMSM) or a surface permanent magnet synchronous motor (SPMSM) may need to be reduced before being used in an EPS application. This can be done using passive machine design techniques, or by employing an active torque ripple cancellation algorithm. Torque ripple cancellation needs a pulsating voltage command in order to generate the cancellation current or torque signal. The amplitude of this pulsating voltage is a function of the synchronous frequency or electrical velocity of the electric motor. As the motor control system approaches the supply voltage limit, the final voltage command must be limited to the maximum available supply voltage. This is because the saturation of voltage commands result in degraded performance of the overall drive system.
The ramp-down of the torque ripple cancellation command is typically achieved through an offline lookup table as a function of synchronous frequency and motor torque or currents. The table is populated based on the calculations performed using worst-case motor parameters and cancellation commands. This open-loop approach does not always result in an optimal ramp down signal, since it does not take in to account the operating condition of the machine. Furthermore, this technique is also memory intensive.